Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted within a housing, which is positioned on top of a truss or tubular tower. The turbine's blades transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
Many types of generators have been used in wind turbines.
At least one prior art wind turbine has included a doubly-fed wound rotor generator. See U.S. Pat. No. 4,994,684, entitled "Doubly Fed Generator Variable Speed Generation Control System," issued Feb. 19, 1991.
A wound rotor induction generator (WRIG) typically includes four major parts: the stator, the rotor, slip rings, and the end caps with bearings. A cross-sectional view of a two-pole 3-phase generator is shown in FIG. 1 where, for simplicity, the windings are shown as a pair of conductors. Referring to FIG. 1, generator 100 comprises stator 101, rotor 102, and winding phase A for each of the rotor and stator, 103 and 104 respectively. A shaft 105 that couples the blades of the wind turbine through the gear box to generator 100 is also shown.
Referring to FIG. 2, in a WRIG system, stator winding 104 is typically connected to the 3-phase utility power grid, such as 480V, 3-phase grid 201, and the rotor winding 103 is connected to a generator-side inverter 202 via slip rings (not shown). The winding 104 is also coupled to the 480V, 3 phase source 201 in parallel with a line-side inverter 203. The line-side inverter 203 and generator-side inverter 202 are coupled together by DC bus 204. The configuration shown in FIG. 2 (i.e., line-side inverter 203, DC bus 204, and generator-side inverter 202) allows power flow into or out of the rotor winding 103. Both inverters are under the control of a digital signal processor (DSP) 205.
Many conventional wind turbines rotate at a constant speed to produce electricity at a constant frequency, e.g., sixty cycles per second (60 Hz), which is a U.S. standard for alternating current or at 50 Hz which is a European standard. Because wind speeds change continuously, these wind turbines utilize either active (pitch regulation) or passive (stall regulation) aerodynamic control in combination with the characteristics of conventional squirrel cage induction generators for maintaining a constant turbine rotor speed.
Some turbines operate at variable speed by using a power converter to adjust their output. As the speed of the turbine rotor fluctuates, the frequency of the alternating current flowing from the generator also varies. The power converter, positioned between the generator and the grid, transforms the variable-frequency alternating current to direct current, and then converts it back to alternating current having a constant frequency. The total power output of the generator is combined by the converter (total conversion). For an example of such a turbine, see U.S. Pat. No. 5,083,039, entitled "Variable Speed Wind Turbine", issued Jan. 21, 1992.
Using variable speed wind turbines to generate electrical power has many advantages that include higher propeller efficiency than constant speed wind turbines, control of reactive power--VARs and power factor, and mitigation of loads.
Some prior art variable speed wind turbines are total conversion systems that use a power converter to completely rectify the entire power output of the wind turbine. That is, the wind turbine, operating at a variable frequency, generates a variable frequency output and converts it into a fixed frequency for tracking the grid. Such systems that utilize total conversion are very costly. Because of the cost, parties are often seeking lower cost solutions, such as for example, a wound rotor generator system utilizing partial conversion in which only a portion of the wind turbine output is rectified and inverted by the power converter.
Some problems currently exist with various control algorithms used by the power converters to control the partial conversion process. For instance, certain systems have stability problems in that they have large oscillations in power and torque. Other systems cannot produce enough power without overheating critical components or are not easily refined to provide a cost effective solution for series production.
Thus, a need exists for a low cost wind turbine system that does not have the stability problems of the prior art, yet still produces a large amount of power, cost effectively, without generating excessive amounts of heat and can be easily refined into a cost effective, readily producible design.